Semiconductor devices made of a nitride semiconductor such as GaN, or a wide-gap semiconductor such as SiC, diamond and ZnO provide a higher breakdown voltage, lower resistance and a higher-speed operation in comparison with the silicon devices. Particularly, GaN or InAlN-type FETs (Field Effect Transistors) so called HEMTs (High Electron Mobility Transistors) and diodes have a hetero-structure, and may include an electron channel with two-dimensionally electron gas (hereinafter, 2DEG) at the hetero-interface. The high electron mobility and high electron density in the 2DEG channel may realize the FETs with the low-resistance and high-speed operation. In other words, it is possible to make a device with a smaller area and a smaller parasitic capacitance than the silicon device, which is conventionally used for power control, and thus to increase the operation speed.
However, p-type semiconductors cannot be easily and flexibly formed in GaN or InAlN-type nitride semiconductors. In addition, the tolerance for the avalanche breakdown is small, when the large transitional voltage is applied. For this reason, the nitride semiconductors adversely affect a circuit design. Concretely, when a voltage more than a rated voltage of FET such as a surge voltage is applied even only for a second, the FET is sometimes broken. Therefore, the nitride semiconductors have room for improvement, in contrast to the silicon FETs that are not immediately broken due to their approximately high avalanche resistance.